In view of the excellent properties of epoxy-based coating compositions, considerable effort has been devoted to developing epoxy-based systems for use in various coating applications, including electrodeposition.
The esterification reaction between certain hydroxy carboxylic acids and epoxy compounds is known in the art. In reacting such compounds, two types of reaction may result due to the chemical nature of the materials used. The hydroxyl groups of the hydroxy acid may react with the epoxide groups to form ether linkages, or the carboxyl group or groups of the acid may react with the epoxide to form ester groups. Both reactions may occur in an uncontrolled reaction to yield products having mixed ether and ester linkages to a non-predetermined degree. Such reaction with the epoxides and acids previously employed have not been tolerable since the ultimate products have not generaly been suitable for any practical purpose.
In an attempt to overcome these problems, the art has attempted to optimize the etherification portion of the reaction while minimizing the esterification portion of the reaction (See, e.g., U.S. Pat. Nos. 3,404,018 and 3,410,773). Similarly, the art has attempted to utilize products containing both ester and ether linkages (See, e.g., U.S. Pat. Nos. 3,707,526 and 3,792,112.) The composition produced using the above techniques suffer from certain disadvantages, including poor cured film saponification resistance, low hydrolysis resistance, and lack of adequate package stability.